Inkjet digital printing has been widely used in many applications. Its refined image quality, comparing with conventional analog technologies such as offset printing, screen printing, lithographic printing, and digital inkjet printing technologies generate much convenient, efficient and environmentally clean results.
However, aqueous digital printing on materials other than paper may be inferior to other processes due to lack of color intensity and speed due to some shortcomings of digital inkjet printing methods. Among those, low viscosity, low colorant content aqueous inks with especially small droplet size requires much more ink materials to generate comparable final image output. This is a greater problem when the colorants used in the inkjet ink are not soluble dyes, but are pigments or other insoluble colorants. High color saturation and so called super saturation of the color image may be difficult to achieve when using these inkjet inks.
One means of creating aqueous ink is adding viscosity control agents that are high molecular weight natural synthetic polymers, higher viscosity water soluble or miscible glycols, high alcohols, accompanied by higher concentrations of colorants. Several problems are associated with such a simple approach. The use of high molecular weight polymers may result in an aqueous system that deviates from physical property requirements of the inkjet printer used to print the ink, such as Newtonian fluid behavior; therefore, the ink improperly responds to the jetting mechanism. The high molecular weight polymer in combination with increased levels of colorants, especially non-soluble type of colorants, can create clogging of the print head nozzles, even when using newer print head technology designed for higher viscosity inks, since such printers are not specifically designed for use with colorants that are solids when printed.
Heat activated colorants have been used in digital inkjet printing. The image quality is dependent on how effectively and efficiently the heat activated colorants are transferred or fixed to the substrate. Hale et al., U.S. Pat. No. 5,642,141 and Xu et al., U.S. Pat. No. 5,488,907 teach inkjet printing methods using finely divided heat activated dye solids. These methods incorporate an ink having a viscosity generally around 2 to 4 cP at ambient temperature. These patents do not specifically teach one how to create a high transfer efficiency ink using heat activated dyes and having relatively higher viscosity.
High viscosity inkjet inks may impose further problems for heat activated inks when a high concentration of colorant is present. Agents for controlling or modifying physical properties of the ink may hinder the heat activation efficiency of the colorant, due to high boiling points, affinity for the heat activated dye at the activation temperature, or entrapment/encapsulation of the dye particle due to the long polymeric chain structure of the chemical/agent. These issues may be more pronounced when the particle sizes of the colorants used in the inks are very small. For example, a high concentration of glycerin may alter the heat activation efficiency of a small dye particle under normal heat activation temperature and duration. Further, a thickening agent, such as carboxy methyl cellulose (CMC), may create a non-Newtonian system, while also hindering activation or sublimation of the heat activated dye.
Ink jet printer print heads, including Drop on Demand (DOD) piezoelectric print heads, have nozzles and orifices of varying sizes. These nozzles and orifices dictate droplet size, print speed, and jettable ink viscosity, and also the tolerance to non-soluble colorants or polymeric particulates. An appropriate range of dye particle sizes based on the nozzle or orifice size is important when formulating the higher viscosity heat activated inks.
There is a need for a higher viscosity ink that comprises heat activated dye solids for digital printing, including transfer printing or direct printing, that will not clog the print head, will yield high heat activation efficiency, and be environmentally safe, and which is suitable for high viscosity ink printers, which is printers that require liquid inks having a viscosity of 5.0 centipoise or greater at ambient temperature.